The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community and is the subject of numerous patents.
U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e. aromatic, unsaturated, saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons e.g. aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain cellulose ester non-porous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
Beyond this, however, the selective removal of aromatics from lube oil distillate streams is also an important processing step in the production of lube and specialty oil base stocks. It is important to remove aromatics from such stocks in order to improve the viscosity index and oxidation/UV stability of the oil. Typically, the aromatics are removed from such lube-specialty oil distillates by solvent extraction using selective extraction solvents such as phenol, furfural, SO.sub.2 or N-methyl-2-pyrrolidone (NMP). While these solvents are professed as being selective, their selectivity is between aromatic hydrocarbons on the one hand and non-aromatic hydrocarbons (e.g. saturates) on the other. In using such selective solvents some good lube molecules such as alkyl single ring aromatics are removed from the lube oil and wind up in the aromatics containing extract phase This results in a loss of yield as well as in a loss of beneficial characteristics the alkyl benzenes could impart to the lube oil insofar as alkyl benzenes have very high viscosity indices and are selectively resistant to oxidation. NMP typically removes multi-ring aromatics first followed by naphthenoaromatics, 1-ring aromatics and paraffins/isoparaffins/naphthenes, in that order. It is difficult to clearly and exclusively remove the multi-ring aromatics. The extract recovered from the extraction process typically contains, in addition to a high concentration of multi-ring aromatics, an appreciable concentration of naphthene aromatics, single ring aromatics and some paraffins/isoparaffins/naphthenes. Thus, as previously stated, extraction results in the loss of some valuable lube molecules to the extract
Thus, the selective removal of multi-ring aromatics from lube oil-specialty oil distillate fractions in a straight forward manner without resort to exotic solvent systems or complicated distillation would be highly attractive and a significant advantage to the industry.
Copending applications U.S. Ser. No. 108,822, filed Oct. 14, 1987 and OP-No. 3463 filed Apr. 11, 1989, both of which are filed in the name of Robert C. Schucker, are directed to aromatic polyurea-urethane membrane which is a symmetric, dense membrane characterized by possessing a urea index, defined as the percentage of the total of urea and urethane groups that are urea, of at least about 20% but less than 100%, an aromatic carbon content of at least about 15 mole %, a functional group density of at least 10 per 1000 grams of polymer, and a C=O/NH ratio of less than about 8. The membrane is shown as being useful for the separation of aromatic from non-aromatics, such as in upgrading aromatics containing streams in petroleum refineries, such streams being, for example, naphtha streams, heavy cat. naphtha streams, intermediate cat. naphtha streams, light aromatics streams boiling in the C.sub.5 -300.degree. F. range, LCCO boiling in the 400.degree.-650.degree. F. range and in chemical applications, i.e. aromatics containing primarily short (i.e. methyl or ethyl) side chains. Those applications do not reveal that the membrane can be used to selectively remove alkyl substituted and alkyl/hetero-atom substituted multi-ring aromatics from lube oil-specialty oil distillate while substantially leaving the alkyl benzenes in the lube oil fraction.
Experimental work reported in Ser. No. 108,822, Examples 10 through 16, showed that selectivity to aromatics over paraffins correlates well with the mole percent aromatic carbon in the molecule. Selectivity to naphthalene (unsubstituted) over paraffins at 80.degree. C. under perstraction conditions is higher than toluene or p-xylene because naphthalene is more aromatic.
By comparison, in the present application the feeds being separated are virgin lube stocks (i.e. distillates) wherein the aromatics have long alkyl side chains or alkyl/hetero-atom side chains. Perstraction involves the selective dissolution of particular components contained in a mixture into the membrane, the diffusion of these components through the membrane and the removal of the diffused components from the downstream side of the membrane by use of a liquid sweep stream. The aromatic molecules present in the feedstream dissolve into the membrane film due to similarities between the membrane solubility parameter and those of the aromatic species in the feed. Another expected constraint on the ability of molecules to dissolve or penetrate into the polymeric film, is the molecular size of the molecules. The size of molecules able to penetrate the film would be limited by the interchain spacing of the polymer. Molecules past a certain molecular weight would be unable to penetrate the film. Thus, it is unexpected that molecules with very high molecular weights of 600 g/mole and higher can penetrate into and diffuse across these membranes. Furthermore, the mole percent aromatic carbon in these molecules is definitely lower than 100% and probably lower than the 75% associated with the xylenes. The mole percent aromatic carbon is most probably lower than 50% because it is known that the aromatic molecules bear one or more alkyl side chains which can be 10 to 12 carbons long. Thus, it would be totally unexpected that molecules having these lower levels of aromaticity would still be more selectively permeated through the membranes than are the saturate molecules and even more unexpected that long alkyl side chain and alkyl/heteroatom side chain substituted multi-ring aromatics having lower mole % aromatic carbon than xylenes would be more selectively permeated through the membrane than the xylenes.